Novel Zn metal–organic framework with the thiazole sites for fast and efficient removal of heavy metal ions from water

Pollution of water by heavy metal ions such as Pb2+ and Hg2+ is considered as an important issue, because of the potential toxic effects these ions impose on environmental ecosystems and human health. A new Zn-based metal–organic framework, [Zn2(DPTTZ) (OBA)2] (IUST-2), was synthesized through a solvothermal method by the reaction of 2, 5-di (4- pyridyl) thiazolo [5, 4-d] thiazole ligand (DPTTZ), the “V-shape” 4,4'-oxybis (benzoic acid) ligand (OBA) and zinc nitrate (Zn(NO3)2·6H2O). This novel MOF has been characterized by several analysis techniques such as fourier transform infrared spectroscopy (FT-IR), elemental analysis (EA), powder x-ray diffraction (PXRD), thermogravimetry analysis (TGA), differential thermal analysis (DTA), field emission scanning electron microscopy (FE-SEM), Brunauer–Emmett–Teller (BET) surface area analysis and single-crystal X-ray diffraction (SXRD). This 3D MOF was tested for removing Pb2+ and Hg2+ ions from water. The factors that were investigated on the elimination of Pb2+ and Hg2+ ions were of pH, adsorption time, and the effect of initial ions concentration. According to the results, this particular Zn-MOF had significant performance in eliminating Pb2+ and Hg2+ ions from water with a removal efficiency of more than 97% and 87% within 3 min, respectively.


Scientific Reports
| (2023) 13:11430 | https://doi.org/10.1038/s41598-023-38523-w www.nature.com/scientificreports/ Experimental Materials and apparatus. All chemicals used in this research were of analytical laboratory grade, provided by well-known commercial sources, and used as received. 2, 5 -di (4-pyridyl) thiazolo [5, 4-d] thiazole (DPTTZ) was synthesized by reported procedure 21 . Powder X-ray diffraction (PXRD) patterns were recorded on Philips X'pert diffractometer. Elemental analysis was performed with a CHNS Thermo Finnigan Flash 1112 series elemental analyzer. The absorption spectroscopy FT-IR was measured in the 400-4000 cm −1 range, by the KBr disc technique on a Shimaduz FT-IR-8400 spectrometer. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out with Perkin Elmer Pyris 1 thermo gravimeter at 10 °C.min −1 heating rate under the argon (Ar) atmosphere. Field emission scanning electron microscopy (FE-SEM) images were taken on the FE-SEM TESCAN MIRA3 microscope. Atomic absorption spectrometry (AAS) on a Shimadzu 6300 AA instrument was used to detect the metal ion concentration in aqueous solutions. Nitrogen adsorption-desorption measurements (BET method) were performed at liquid nitrogen temperature (− 196 °C) with a Micromeritics ASAP 2020 adsorption instrument.
Single crystal X-ray diffraction. Crystallographic data for the IUST-2 were collected using Mo Kα radiation (λ = 0.71073 Å) on a Marresearch 345 dtb diffractometer equipped with an image plate detector. The programs used to solve and refine the structure were SHELXT 2018/2 (Sheldrick, 2018) and SHELXL2016/6 (Sheldrick, 2016), respectively 22 . Data reduction and cell refinement were carried out with the Automar software package (3.3a, 2015). Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were added at ideal positions and refined using a riding model. Table S1 summarized the single-crystal x-ray diffraction data and structure refinement for IUST-2. To investigate the kinetics of the sorption process, 5 mg IUST-2 was placed in a round-bottomed flask and then left to interact with 25 mL of aqueous solutions at 200 mg L −1 concentration of Pb 2+ and Hg 2+ , separately, for various time intervals between 0.5 to 120 min. Moreover, the maximum adsorption capacities of IUST-2 for lead and mercury ions were estimated separately, by adsorption isotherms in a series of Pb 2+ and Hg 2+ solutions with the concentration range of 50-600 mg L −1 . In all the aforementioned tests, pH values were set to be 5 and 4 for lead and mercury solutions, respectively.

Results and discussion
Crystal structure and characterization of IUST-2. According to the single crystal X-ray analysis, IUST-2 crystallizes in an orthorhombic crystal system and the space group is Pbcn with the asymmetric unit comprising of two Zn 2+ ions, one DPTTZ ligand and two OBA ligands. As illustrated in Fig. 1a (17)°, respectively. From a crystallography point of view, four carboxyl groups from four OBA ligands, act as a bridging ligands to link independent Zn 2+ ions, making a paddle-wheel shaped secondary building unit (SBU) Zn 2 (CO 2 ) 4 . The carboxylate groups of OBA ligands adopting bismonodentate coordination modes μ 2 -η 1 :η 1 link Zn nodes to form a two-dimensional wave-like sheets expanded by DPTTZ ligands, eventually resulting a 3D structure ( Fig. 1b and c).
PXRD. The phase purity of IUST-2 was checked by powder X-ray diffraction (PXRD) experiments. The simulated and experimental PXRD patterns of the IUST-2 correspond to each other, demonstrating the phase purity of the IUST-2 crystals prepared using the solvothermal method. Comparison of powder x-ray diffraction patterns before and after the activation process with methanol showed that the IUST-2 structure remains intact after activation (Fig. S1). Considering the target application for IUST-2 as an adsorbent to remove metal ions in aqueous media, the water-stability of the IUST-2 was further studied. The IUST-2 was immersed in water at RT for 48 h. Slight changes in the PXRD pattern were observed after 48 h of soaking in water, which returned to TGA and DTA. In order to investigate the thermal stability of IUST-2, thermogravimetric and differential thermal analysis experiments were carried out under the Ar atmosphere (Fig. S3). A weight loss with a mild rate (approximately 20%) in the beginning, corresponds to the evaporation of trapped solvent molecules inside the IUST-2 during the washing process. Decomposition and pyrolysis of the IUST-2 starts with an endothermic process at 363 °C, which was the main thermal loss of 72%. The remaining weight (approximately 8%) may be attributed to the formation of zinc oxide, or zinc sulfide. This indicates that IUST-2 has rather good thermal stability.

BET.
To measure N 2 adsorption/desorption isotherm, a sample IUST-2 was degassed at 110 °C for 4 h. Fig. S4 shows that the N 2 adsorption-desorption isotherm of the IUST-2 is of type IV, according to the IUPAC classification, having an H3 type hysteresis loop. The Brunauer-Emmett-Teller (BET) specific surface area of the IUST-2 is 105.636 m 2. g −1 . The pore volume and average pore size are 0.081 cm 3. g −1 and 30.553 Å, respectively. Figure S5 shows the FE-SEM images of IUST-2 obtained by the solvothermal process. As could be observed from these figures, IUST-2 has a nanostructure with an average diameter of 31.4 nm.

Selective adsorption behaviors. Batch sorption tests on an aqueous solution with a range of different
heavy metal ions such as (Hg 2+ , Pb 2+ , Cu 2+ , Cd 2+ , Ni 2+ , Co 2+ ) were carried out to determine the capacity and behavior of IUST-2 in selective adsorption. From the results, it can be noticed that IUST-2 shows much higher removal efficiency in the presence of the Pb 2+ (%97.08) and Hg 2+ (%87.55) compared to other heavy metal ions, when applied to single solutions of different ions (Fig. 2a). In another test, and to investigate the interfering effects, the removal efficiency was studied by the IUST-2 on a mixed aqueous solution of different heavy metal ions, with a concentration of 100 mg . L −1 each. Still, the removal efficiency was much higher for Pb 2+ and Hg 2+ (88% and 79%, respectively), even in the presence of other interfering ions in the solution (Fig. 2b). It can be observed that IUST-2 revealed excellent uptake performance toward lead and mercury ions, which is assumed to be in relevance to thiazole ring. Thiazole is a unique heterocyclic compound containing sulfur and nitrogen  The effect of concentration. The influence of initial Hg 2+ and Pb 2+ concentrations on the adsorption efficiency by the IUST-2 was examined at room temperature. The adsorption isotherm curves are demonstrated for the adsorption of Pb 2+ (Fig. 4a) and Hg 2+ ions (Fig. 4b) at different initial concentrations (50-600 mg L −1 ) by 5 mg of the IUST-2 in 25 mL solution of heavy metal ions. It was observed that the adsorption capacity of the IUST-2 for Pb 2+ and Hg 2+ increased gradually to 1450 mg g −1 and 900 mg g −1 , respectively, at an initial concentration of 500 mg L −1 . These values for adsorption capacity are so much larger compared to other reported values for other MOFs [29][30][31][32][33] .
The experimental adsorption isotherms data fitted well with the Langmuir model, which is evident from the high correlation coefficient (R 2 = 0.9933 for Pb 2+ and 0.9980 for Hg 2+ ) values. The equation of the Langmuir isotherm is given as Eq. (1): where q t (mg g −1 ) represents the adsorption capacity at time t, c t (mg L −1 ) is the metal ion concentration at time t, b (L mg −1 ) is related to the energy of adsorption that demonstrates the affinity between the solute and the adsorbent, and q e (mg g −1 ) represents the maximum monolayer adsorption capacity.  www.nature.com/scientificreports/ Since the Langmuir model is compatible to the adsorption behavior, it is concluded that the distribution of active sites on the surface of IUST-2 is homogeneous 34,35 . The results of the Langmuir model for lead (Fig. 4c) and mercury (Fig. 4d) ions adsorption on the IUST-2 is exhibited in Table S2. The theoretical maximum monolayer adsorption (q e ) of Pb 2+ and Hg 2+ ions were computed to be 1430 and 900 mg g −1 , respectively, which are very close to the values of experimental maximum adsorption capacity (1450 and 900 mg.g −1 ). Adsorption kinetics and mechanism. As shown in Fig. 5a and b, variation of the removal capacity for Pb 2+ and Hg 2+ ions on the IUST-2 with time were achieved. Due to the presence of specific functional groups on the IUST-2 and high metal ions affinity for the sulfur and nitrogen of the thiazole ring, the removal of targeted ions is very quick and the maximum adsorption capacity is acquired after 3 min. The activated IUST-2 (5 mg) was placed in an aqueous solution of Pb(NO 3 ) 2 and HgCl 2 (25 mL, 100 mg L −1 initial concentration, pH = 5 and 4 for lead and mercury ions, respectively) and then the achieved data were fitted with pseudo-second order kinetic model (Fig. 5c for lead and Fig. 5d for mercury ions) using the Eq. (2): where q e and q t (mg . g −1 ) are the removal capacity at equilibrium and time t (min), respectively; k 2 (g (mg min) −1 ) is the pseudo-second order rate constant of the adsorption rate. The high coefficient of determination (R 2 ˃ 0.9999) value was obtained, suggesting that the pseudo-second-order model was suitable for the adsorption kinetics of the IUST-2 and sorption process was mostly governed by chemical reactions between the metal ions and the active adsorption sites of the IUST-2 (Table S3) 36,37 .
To examine the mechanism of Hg 2+ and Pb 2+ ions sorption, the FT-IR spectrum of IUST-2 before and after the adsorption of metal ions was studied (Fig. 6). There are obvious changes in the infrared spectra of C − S and C − N vibrations of thiazole ring. There are strong interactions between Hg 2+ and Pb 2+ ions with S and N atoms of the thiazole ring that could limit C − S and C − N vibrations and consequently decrease their vibrational frequency. The peak at 833 cm −1 assigned to C − S stretching vibration that showed a red shift of the 833 cm −1 peak to 820 cm −1 peak and 824 cm −1 peak after treatment with Pb and Hg ions, respectively. Moreover, a significant red shift from 1407 cm −1 to 1392 cm −1 and 1384 cm −1 was observed for the characteristic C − N stretching vibration after the adsorption process, for lead and mercury ions, respectively, which confirmed the coordination of Pb 2+ and Hg 2+ ions to the nitrogen of the thiazole ring. Also, a new bond has appeared in 542 cm −1 , which can attribute to the Pb-O vibration 38 . The Hg-O bond for mercury does not exist in the FT-IR spectrum, so the reason for the increase in the percentage of removal efficiency of IUST-2 for lead compared to mercury can be attributed to the presence of more adsorption sites for lead. The results of the powder x-ray diffraction before and after the adsorption process reveal that the IUST-2 can retain its crystallinity and structure after the adsorption of Pb 2+ and Hg 2+ metal ions, so the possibility of structural collapse should be removed (Fig. S6). www.nature.com/scientificreports/ Reusability. As a general rule, reusability and stability are two essential factors for adsorbent to work effectively. When the adsorption process was finished, the IUST-2 was regenerated by using an EDTA.2Na solution. Three cycles of adsorption/desorption were carried out to evaluate the reusability of the IUST-2 and this process was monitored with AAS. Figure 7 demonstrates that the IUST-2 possesses reversibility in the process of removing Pb 2+ and Hg 2+ ions.    (Table S4). Also, a decrease was observed in the total pore volume from 0.081 to 0.020 cm 3 /g, and 0.066 cm 3 /g for the IUST-2 after the adsorption of Pb (II) and Hg (II) ions, respectively (Table S4). This might result from filling the volume of the pores. Hence, it can be concluded that IUST-2 is a good absorbent towards lead and mercury ions from the aqueous solution. In addition, the results of the BET surface area and pore volume after the removal of metal ions confirmed the reusability of IUST-2.
Comparison with other MOF-based adsorbents. Table 1 indicates the maximum sorption capacity of IUST-2 for the removal of lead and mercury ions from water compared to other MOF-based adsorbents in the literature. According to Table 1, the maximum sorption capacities of IUST-2 for Pb (II) and Hg (II) are higher than that of most other MOF-based adsorbents reported in the literature. Totally, these results demonstrated that IUST-2 is a promising adsorbent for the effective removal of Pb (II) and Hg (II) ions from polluted water.

Conclusions
In conclusion, metal-organic frameworks containing the thiazole ring can be a good option for the adsorption of heavy metal ions. In this article, a new Zn-MOF, the IUST-2, based on the thiazole ligand was synthesized by solvothermal method. According to the single crystal x-ray diffraction, the IUST-2 is a 3D bipillared-layer framework structure. The IUST-2 displayed remarkable application in the adsorption of lead and mercury ions from water. The removal efficiency as high as 97% and 87% was obtained for Pb 2+ and Hg 2+ ions at an initial concentration of 100 mg L −1 after 3 min, respectively. Maximum adsorption capacity is 1450 mg g −1 for Pb 2+ and 900 mg g −1 for Hg 2+ ions. The results from Langmuir and pseudo-second order rate models reveal the removal of metal ions by the IUST-2 is a monolayer adsorption by the interaction between the active adsorption sites of the IUST-2 and Pb 2+ and Hg 2+ ions. This study indicates that metal-organic frameworks based on thiazole ligands can be good adsorbents for mercury and lead ions elimination of aqueous solution.

Data availability
All data generated or analyzed during this study are included in this published article [